Capacitor and method for producing a capacitor

ABSTRACT

An electrode for an electrolytic capacitor includes a substrate comprising titanium; a carbide layer adjacent the substrate; and a carbonaceous layer adjacent the carbide layer and including means for enhancing the capacitance of the electrode.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/003,183 filed Dec. 3, 2004, which is a continuation-in-part of bothU.S. patent application Ser. No. 10/448,556 filed May 30, 2003 and U.S.patent application Ser. No. 10/260,682 filed Sep. 30, 2002. The entiredisclosures the following applications are expressly incorporated byreference herein in their entirety: U.S. patent application Ser. No.11/003,183; U.S. patent application Ser. No. 10/448,556; and U.S. patentapplication Ser. No. 10/260,682.

This application is also related to U.S. patent application Ser. No.10/449,879 and U.S. patent application Ser. No. 10/449,645, each ofwhich were filed on May 30, 2003.

BACKGROUND

The present invention relates generally to the field of capacitors. Morespecifically, the present invention relates to electrolytic capacitorsfor use in a device such as an implantable medical device.

Since their earliest inception, there has been significant advancementin the field of body-implantable electronic medical devices. Today, suchimplantable devices include therapeutic and diagnostic devices, such aspacemakers, cardioverters, defibrillators, neural stimulators, drugadministering devices, and the like for alleviating the adverse effectsof various health ailments.

Implantable medical devices may utilize a capacitor to perform variousfunctions. For example, if the implantable medical device is adefibrillator, one or more capacitors may be used to provide atherapeutic high voltage treatment to a patient.

One type of capacitor that may be used in such an application is anelectrolytic or wet slug capacitor. Conventional wet slug capacitors mayinclude a container formed from tantalum or a tantalum alloy that actsas the cathode. An electrolyte (e.g., an acid such as phosphoric acid)and an anode are provided within the container. In these types ofcapacitors, an anodic oxide may be formed on exposed surfaces.

Since the electrolyte is electrically conductive, aconductor-insulator-conductor structure including metal, oxide coating,and electrolyte is present at both the anode and the cathode. Each ofthese conductor-insulator-conductor structures constitute themselves acapacitor.

In the conventional wet slug capacitor, the anode capacitance iselectrically connected in series with the cathode capacitance. The totalcapacitance C_(total) of the two capacitors C_(anode) and C_(cathode) inseries is expressed by the formula1/C_(total)=1/C_(anode)+1/C_(cathode). In order to maximize C_(total),the capacitance C_(cathode) has to be as large as possible.

Although conventional wet slug capacitors having useful capacitanceshave been produced, there is a desire to increase the energy per unitvolume of capacitor anode (i.e., the stored energy density). The energyE stored inside a capacitor may be expressed by the formulaE=½C_(total)U², where U is the potential to which the capacitor ischarged. Hence, high energy density requirements demand high-capacitanceper-unit area cathodes so as to maximize C_(total) and, in turn, E.Conventional capacitor cathode materials (e.g., tantalum), however, mayprovide a limited capacitance per unit area. For certain applications,it is desirable to provide a capacitor cathode that has a capacitance inthe range of approximately 10-20 milliFarads per square centimeter ofgeometrical surface area.

Accordingly, there is a need to provide an electrode for a capacitorthat utilizes a material which enhances the capacitance for theelectrode relative to conventional capacitor electrodes (e.g., providesa capacitor electrode having a specific capacitance of greater thanapproximately 10 milliFarads per square centimeter). It would bedesirable to provide a method of producing such an electrode using amethod which is relatively simple in terms of the processing involvedand that does not adversely affect capacitor performance. There is alsoa need for an electrode that utilizes a material which produces arelatively smooth and defect-free electrode surface. There is further aneed to provide a capacitor that includes at least one electrode thatexhibits increased capacitance as compared to conventional capacitorelectrodes.

It would be desirable to provide an electrode for a capacitor and acapacitor that provides one or more of these or other advantageousfeatures. Other features and advantages will be made apparent from thepresent description. The teachings disclosed extend to those embodimentsthat fall within the scope of the appended claims, regardless of whetherthey provide one or more of the aforementioned advantages.

SUMMARY

An exemplary embodiment of the invention relates to an electrode for anelectrolytic capacitor that includes a substrate comprising titanium; acarbide layer adjacent the substrate; and a carbonaceous layer adjacentthe carbide layer and including means for enhancing the capacitance ofthe electrode.

Another exemplary embodiment of the invention relates to a capacitorthat includes a cathode, an anode, and an electrolyte providedintermediate the cathode and the anode. The cathode includes a substratecomprising titanium, a carbide layer adjacent the substrate, and acarbonaceous layer adjacent the carbide layer comprising a material forenhancing the capacitance of the electrode.

Another exemplary embodiment of the invention relates to an implantablemedical device that includes an electrolytic capacitor having a cathode,an anode, and an electrolyte intermediate the cathode and the anode. Thecathode includes a substrate comprising titanium, a carbide layeradjacent the substrate, and a carbonaceous layer adjacent the carbidelayer comprising a material for enhancing the capacitance of theelectrode. The implantable medical device is configured to provide atherapeutic high voltage treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in the following textwith reference to the attached drawings, in which:

FIG. 1 is a schematic drawing showing an implantable medical deviceshown in the form of a defibrillator implanted within a human body;

FIG. 2 is a schematic drawing of a capacitor bank for use with theimplantable medical device shown in FIG. 1;

FIG. 3 is a schematic drawing showing the capacitor bank shown in FIG. 2coupled to a battery;

FIG. 4 is a schematic cross-sectional view of one of the capacitorsprovided within the capacitor bank shown in FIG. 2 according to anexemplary embodiment;

FIG. 5 is a schematic cross-sectional view of one of the capacitorsprovided within the capacitor bank shown in FIG. 2 according to anotherexemplary embodiment;

FIG. 6 is a schematic cross-sectional view of a portion of a capacitoraccording to an exemplary embodiment;

FIG. 7 is a schematic cross-sectional view of the portion of thecapacitor shown in FIG. 6 showing a layer of carbon provided over asubstrate;

FIG. 8 is a schematic cross-sectional view of the portion of thecapacitor shown in FIG. 6 showing a carbide formation step;

FIG. 9 is a schematic cross-sectional view of the portion of thecapacitor shown in FIG. 6 showing the formation of acapacitance-enhancing layer;

FIG. 10 is a flow diagram illustrating a process for forming the portionof the capacitor shown in FIG. 6;

FIG. 11 is a graphical representation showing the capacitance versesfrequency functionality for electrodes having a titanium substrate;

FIG. 12 is a micrograph showing an electrode surface having a damaged orruptured characteristic; and

FIG. 13 is a micrograph showing an electrode surface having a relativelysmooth characteristic.

DETAILED DESCRIPTION

With reference to the accompanying FIGURES, the present disclosurerelates to capacitors (e.g., electrolytic capacitors) for use in medicaldevices (e.g., implantable medical devices), methods of producing suchcapacitors, and medical devices which utilize such capacitors. While thesubject matter herein is presented in the context of the use of suchcapacitors in the field of implantable medical devices, such capacitorsmay be utilized in alternative applications such as external medicaldevices or other devices utilizing a capacitor, as will be appreciatedby those of skill in the art who review this disclosure.

Referring to FIG. 1, a system 10 including an implantable medical device(IMD) is shown as being implanted within a body or torso 32 of a patient30. The system 10 includes a device 12 in the form of an implantablemedical device that for purposes of illustration is shown as adefibrillator. The defibrillator is configured to provide a therapeutichigh voltage (e.g., 700 volt) treatment for the patient 30. While theimplantable medical device is shown and described as a defibrillator, itshould be appreciated that other types of implantable medical devicesmay be utilized according to alternative embodiments. According to stillother alternative embodiments, non-implantable (e.g.,externally-connected) medical devices or other types of devices mayutilize capacitors as are shown and described in this disclosure.

The device 12 includes a container or housing 14 that is hermeticallysealed and biologically inert according to an exemplary embodiment. Thecontainer may be made of a conductive material. One or more leads 16electrically connect the device 12 and the patient's heart 20 via a vein22. Electrodes 17 are provided to sense cardiac activity and/or providean electrical potential to the heart 20. At least a portion of the leads16 (e.g., an end portion of the leads shown as exposed electrodes 17)may be provided adjacent or in contact with one or more of a ventricleand an atrium of the heart 20.

A capacitor bank 40 including a plurality of capacitors is providedwithin the device 12. A schematic view of the capacitor bank 40 is shownin FIG. 2, and shows a group of five capacitors 42 connected in seriesand provided within the capacitor bank. The size and capacity of thecapacitors may be chosen based on a number of factors, including theamount of charge required for a given patient's physical or medicalcharacteristics. According to other exemplary embodiments, the capacitorbank may include a different number of capacitors (e.g., less than orgreater than five capacitors). According to still other exemplaryembodiments, a different number of capacitor banks may be providedwithin the implantable medical device having any suitable number ofcapacitors provided therein.

As shown in FIG. 3, the capacitor bank 40 is coupled to a battery 50.According to an exemplary embodiment, the battery 50 is included withinthe device 12. According to alternative embodiments, the battery may beprovided external to the device 12. The capacitors 42 provided withinthe capacitor bank are configured to store energy provided by thebattery 40. For example, the system 10 may be configured such that whenthe device 12 determines that a therapeutic high-voltage treatment isrequired to establish a normal sinus rhythm for the heart 20, thecapacitors 42 in the capacitor bank 40 are charged to a predeterminedcharge level by the battery 50. Charge stored in the capacitors 42 maythen be discharged via the leads 16 to the heart 20. According toanother exemplary embodiment, the capacitors may be charged prior todetermination that a stimulating charge is required by the heart suchthat the capacitors may be discharged as needed.

Various types of capacitors may be provided within the capacitor bank 40according to various exemplary embodiments. FIG. 4 shows a schematiccross-sectional view of a portion of a capacitor 60 according to a firstexemplary embodiment. The capacitor 60 includes a container or housing62 (e.g., a hermetically sealed container). According to an exemplaryembodiment, the container comprises titanium. According to otherexemplary embodiments, other materials may be used in place of or inaddition to titanium (e.g., tantalum, niobium, zirconium, etc.).

A cathode 68 is provided within the container 62. According to anexemplary embodiment, the cathode 68 is electrically isolated from aninner surface 64 of the container 62. A cathode lead 70 is electricallycoupled to the cathode 68 and extends through a wall 66 of the container62. The cathode lead 70 is electrically isolated from the container 62by a feed-through 72. According to an exemplary embodiment, thefeed-through 72 comprises an insulating material (e.g., glass) thatseals the cathode lead 70 from the container 62. The feed-through 72 mayalso act to prevent material (e.g., electrolyte) from escaping thecontainer 62 and to prevent foreign matter from entering the container62 in the location of the cathode lead 70.

An anode 78 is provided within the container 62. According to anexemplary embodiment, the anode 78 comprises tantalum. According toother exemplary embodiments, the anode 78 may comprise other materialsin addition to or in place of tantalum (e.g., titanium, niobium,zirconium, etc.). The anode 78 is provided in the container 62 such thatit is not in direct contact with (e.g., is spaced apart from) thecathode 68.

The anode 78 is electrically coupled to an anode lead 74 that passesthrough a wall 66 of the container 62 via-a feed-through 76. Thefeed-through 76 may be constructed in a similar manner as described withrespect to feed-through 72, and may act to electrically isolate theanode lead 74 from the container 62 in substantially the same manner asdescribed with respect to cathode lead 70 and feed-through 72.

An electrolyte 79 (e.g., a fluid or liquid electrolyte) is provided inthe container 62. At least a portion of the electrolyte 79 is providedintermediate the cathode 68 and the anode 78. The electrolyte 79provides negative and positive ions to constitute capacitors at thecathode 68 and the anode 78. According to an exemplary embodiment, theelectrolyte 79 comprises ammonium salts (e.g., ammonium acetate)dissolved in a water and glycol mixture, phosphoric acid, etc. Theparticular electrolyte chosen may depend on a number of factors, such asthe desired conductivity of the electrolyte.

FIG. 5 shows a cross-sectional schematic view of a portion of acapacitor 80 according to another exemplary embodiment. The capacitor 80includes a container or housing 82 which may be constructed in a mannersimilar to that described with respect to container 62.

A cathode 84 is integrally formed with the container 82 such that thecathode 84 forms an inner surface 86 of the container 82. The cathode 84is electrically coupled to a cathode lead 90 that extends from the wall88 of the container 82.

An anode 96 is provided within the container 62 such that the anode 96is not in contact with (e.g., is spaced apart from) the cathode 84.According to an exemplary embodiment, the anode 78 comprises tantalum.According to other exemplary embodiments, the anode 78 may compriseother materials in addition to or in place of tantalum (e.g., aluminum,titanium, niobium, zirconium, etc.).

The anode 96 is electrically coupled to an anode lead 92 through afeed-through 94. The feed-through 94 may be constructed in a similarmanner to that described with respect to the feed-through 72 and thefeed-through 76.

An electrolyte 98 (e.g., a fluid or liquid electrolyte) is provided inthe container 82. At least a portion of the electrolyte 98 is providedintermediate the cathode 84 and the anode 96 and provides negative andpositive ions to constitute capacitors at the cathode 84 and the anode96. The electrolyte utilized in the capacitor 80 may be the same as ormay differ from that utilized in the capacitor 60. The particularelectrolyte chosen may depend on a number of factors, such as thedesired conductivity of the electrolyte.

FIG. 6 shows a more detailed schematic cross-sectional view of a portion102 of an electrode 100 (e.g., a cathode or an anode) that may beutilized in a capacitor such as that shown in FIGS. 4 and 5. It shouldbe noted that while two specific capacitor arrangements have been shownand described with respect to FIGS. 4 and 5, the electrode 100 may alsobe used with other types of capacitors without departing from the scopeof this disclosure. Accordingly, the electrode 100 will be describedherein as being configured for use with a capacitor such as anelectrolytic capacitor without being restricted to the particularcapacitor types shown and described herein.

The electrode 100 is provided as a multilayer structure 104 and includesa substrate or base material 110, a carbide layer 120, and a layer ofmaterial 130 (i.e., a carbonaceous or carbon-containing layer) that actsto enhance the capacitance of the electrode 100. In this manner, thecarbonaceous layer 130 may be referred to as a capacitance-enhancingmaterial or layer. An electrolyte 140 is in contact with thecarbonaceous layer 130.

The substrate 110 is a metal selected from titanium, aluminum (for ananode), tantalum, niobium, zirconium, silver, stainless steel, andalloys of any one or more of the foregoing metals. The particularsubstrate material chosen may depend on a variety of factors. Where thesubstrate is part of a cathode, the substrate material is preferablyrelatively tough, weldable, and resistant to chemicals. For example, asubstrate for a cathode may be titanium or a titanium alloy. In otherexamples, the cathode may include a substrate made of tantalum, niobium,or alloys thereof. Other materials may also be used.

Where the substrate is part of an anode, the material chosen preferablyhas the capability to form an anodic oxide thereon. Because such metalsallow electrons generally to travel in one direction but not another(i.e., due to the interface between the metal and an oxide formedthereon), such materials may generally be referred to as valve metals.For example, a substrate for use in an anode may be aluminum, tantalum,niobium, or alloys thereof. Other materials may also be used.

While any of a variety of materials may be used for the substrate,according to a preferred embodiment, the substrate 110 is titanium or atitanium alloy (e.g., titanium or-titanium alloy foil or sheet metal).According to an alternative embodiment (in an anode), the substrate isaluminum or an aluminum alloy. According to various other alternativeembodiments, the substrate may comprise a glassy carbon material or anyother material that may be acceptable for use as an electrode substrate.

The substrate 110 may be provided as a single layer of material or mayinclude multiple layers of material. The various layers may have thesame composition or may differ from each other (e.g., a substrate may beprovided as having alternating layers of titanium and a titanium alloy).It should be noted that the materials described for the substrate 110are not exclusive, and other metals or materials may be utilizedaccording to alternative embodiments.

According to a preferred embodiment, the substrate 110 has a thicknessof between approximately 150 and 250 micrometers. According to variousalternative embodiments, the substrate may have a thickness of betweenapproximately 50 and 500 micrometers.

According to a preferred embodiment, the substrate 110 includes asurface 112 that has a relatively rough characteristic or configuration(e.g., the surface 112 is not entirely planar or flat, and may includeprotrusions or extensions that extend from the surface 112 to form asurface having peaks and valleys). One nonexclusive example of such asurface is shown in FIG. 6, although the precise nature of such asurface may vary according to alternative embodiments. According to apreferred embodiment, the surface 112 has a roughness of betweenapproximately 2 and 5 micrometers (e.g., the average height or distancebetween the peaks and valleys is between 2 and 5 micrometers). It isintended that the roughness of the surface 112 provides for enhancedmechanical bonding between the substrate 110 and a layer of materialprovided in contact with the substrate 110. Chemical bonding may beutilized in addition to mechanical bonding to secure such a layer ofmaterial to the substrate 110.

While it is preferred that the surface 112 has a relatively roughsurface finish to provide for enhanced mechanical bonding, according toan alternative embodiment, the surface of the substrate may have arelatively flat or planar surface finish. In such a case, chemicalbonding may play a larger role in bonding a layer of material to thesubstrate.

According to an exemplary embodiment, the substrate has a relativelythin (e.g., less than approximately 10 nanometers) native oxide layer.For example, according to a preferred embodiment in which the substrate110 is titanium or a titanium alloy, the substrate 110 may include arelatively thin titanium oxide layer on the surface 112 thereof.

The carbide layer 120 is provided adjacent or proximate (e.g., incontact with) the substrate 110. According to an exemplary embodiment,the carbide layer 120 is a metal carbide comprising carbon atoms andmetal atoms as provided in the adjacent substrate 110. For example,according to a preferred embodiment in which the substrate 110 istitanium, the carbide layer 120 is titanium carbide (TiC). Other typesof carbides may also be utilized. In providing the carbide layer 120,oxygen atoms included in the native titanium oxide layer may bedisplaced by carbon atoms in an elevated temperature reaction as will bedescribed in greater detail below. According to an alternativeembodiment, no carbide layer is provided (e.g., where the substratecomprises aluminum or an aluminum alloy, as in an anode application).According to still other alternative embodiments, an additional layer ofmaterial may be provided intermediate or between the substrate and thecarbide layer.

The carbide layer 120 is bonded or coupled both chemically (e.g., is achemisorbed layer) and mechanically (e.g., by virtue of interactionbetween the carbide and the roughened surface 112 of the substrate 110)to the substrate 110 according to a preferred embodiment. According toan alternative embodiment, the carbide layer may be coupled by purelymechanical or purely chemical means to the surface.

The carbide layer 120 is formed by providing a layer ofcarbon-containing material (e.g., graphite powder or particles withindividual particle sizes of about 1 micrometer) adjacent the substrate110. According to a preferred embodiment, the layer of carbon has athickness of between approximately 20 and 30 micrometers and isdeposited as up to 10 or more coats or layers of a graphite powdersuspension in a carrier liquid. According to alternative embodiments,the layer of carbon may be provided as having a different thicknessand/or may be applied in a different number of coats or layers.

An elevated temperature or vacuum baking step is performed in which thecarbon-containing material is subjected to a temperature of betweenapproximately 800° and 1000° C. in a vacuum furnace at a pressure ofapproximately 10E-6 Torr, during which at least a portion of the carbonincluded in the carbon-containing material forms a metal carbide (e.g.,by displacing oxygen atoms in the native oxide formed on the surface ofthe substrate). According to a preferred embodiment, at least a portionof the carbon-containing material is not converted to a carbidematerial, and remains as bulk carbon-containing material adjacent thecarbide layer 120. According to an alternative embodiment, thecarbon-containing material is entirely consumed in the vacuum bakingstep, such that no carbon-containing material is left adjacent thecarbide layer. In yet another alternative embodiment, the vacuum bakingstep may be replaced with an elevated temperature process performed inan inert (e.g., argon) atmosphere.

According to a preferred embodiment, the thickness of the carbide layer120 is less than approximately 10 nanometers. According to alternativeembodiments, the thickness of the carbide layer may be greater than 10nanometers (e.g., between approximately 10 and 500 nanometers).

The carbonaceous layer 130 is provided adjacent or proximate the carbidelayer 120, and provides enhanced capacitance for the electrode 100. Thecarbonaceous layer 130 includes activated carbon (formed from thecarbon-containing material provided adjacent the carbide layer 120) andan oxide of manganese (e.g., manganese dioxide (MnO₂)). The carbonaceouslayer may also include unreacted (e.g., non-activated) carbon-containingmaterial intermediate or between the activated carbon and the carbidelayer. According to an alternative embodiment, the carbon provided inthe carbonaceous layer is not activated, and instead includes carbon(e.g., graphite) and an oxide of manganese.

As described above, according to a preferred embodiment, a portion ofthe carbon-containing material remains adjacent the carbide layer afterthe vacuum baking step that converts a portion of the carbon-containinglayer to a metal carbide material. According to an alternativeembodiment, additional carbon-containing material (either the same as ordifferent from the carbon-containing material used to form the carbidelayer) may be provided adjacent the carbide layer.

The substrate 110 and the carbide layer 120 (and any remaining ordeposited carbon-containing material) may be cooled to a temperature ofbetween approximately 20° and 100° C. and then heated in anoxygen-containing atmosphere or ambient (e.g., air, pure oxygen, etc.)to a temperature of between approximately 400° and 450° C. for a periodof between approximately 30 and 90 minutes. This heating step serves toactivate at least a portion of the carbon-containing material providedadjacent the carbide layer 120 by forming functional groups that includeoxygen. For example, an activated carbon region 134 formed in thecarbonaceous layer 130 may include oxygen-containing functional groupssuch as CO, COOH, and C═O.

The activated carbon region 134 exhibits increased porosity as comparedto the carbon-containing material from which it is formed, which allowsliquid electrolyte to penetrate at least a portion of the carbonaceouslayer 130. One advantageous feature of providing an activated carbonregion 134 is that the surface area of the carbonaceous layer 130 isincreased, which in turn acts to increase the capacitance of theelectrode 100.

The thickness of the activated carbon region 134 is a function of theamount of time that elapses during the elevated temperature activation.According to a preferred embodiment, the thickness of the carbonaceouslayer 130 is between approximately 10 and 50 micrometers, and thethickness of the activated carbon region 134 is between approximately 5and 25 micrometers after activating the carbon-containing material at atemperature of approximately 250° C. for a period of approximately 0.5hours. A layer 136 of unreacted (e.g., non-activated) carbon-containingmaterial remains intermediate or between the activated carbon region 134and the carbide layer 120. According to another embodiment, theactivated carbon portion of the carbonaceous layer extends entirelythrough the carbonaceous layer so that substantially all of thecarbon-containing material is provided as activated carbon.

The carbonaceous layer 130 also includes an oxide of manganese (e.g.,manganese dioxide (MnO₂)). According to a preferred embodiment,manganese nitrate (Mn(NO₃)₂) is provided in solution with water and analcohol, after which the manganese nitrate is heated to provide an oxideof manganese. The solution of manganese nitrate, water, and alcoholpenetrates the relatively porous activated carbon region 134, and uponheating the activated carbon region 134 and the solution, a finedispersion of oxide is provided in the pores of the activated carbonregion 134. It is intended that the inclusion of metal oxide in thecarbonaceous layer 130 acts to further enhance the capacitance of theelectrode 100.

A method or process 200 of preparing the electrode 100 is now describedwith reference to FIGS. 6-9. A flow diagram illustrating such a methodor process 200 is provided as FIG. 10.

As shown in FIG. 6, the substrate 110 is provided in a step 210.According to a preferred embodiment, the substrate 110 is titanium or atitanium alloy and is provided as a foil or sheet of metal. According toalternative embodiments, the substrate may be made of aluminum or avariety of other materials acceptable for use in capacitor electrodes asdescribed above.

In a step 220, a surface 112 of the substrate 110 is altered or deformedto have a relatively rough characteristic or configuration. Variousmethods may be used to provide the surface 112 with its relatively roughsurface finish. For example, according to an exemplary embodiment, agrit blasting technique may be utilized to alter the surface 112. Thegrit may be alumina (AI₂O₃) or silicon carbide (SiC) having a particlediameter of about 1 micrometer. The grit may be accelerated usingcompressed air at pressures between approximately 20 and 40 psi.

According to another exemplary embodiment, an etching process may beutilized to provide the surface 112 with a relative surface finish. Forexample, oxalic acid may be utilized at a temperature of approximately80° C.

According to another exemplary embodiment, the substrate may be providedwith a roughened surface without the need to perform a separateprocessing step. For example, sintered metal particles (e.g., sinteredtitanium) may be deposited on a metal sheet surface (e.g., a titaniumsheet) using a vacuum sintering process.

In a step 230, a carbon layer 115 (e.g., a layer of carbon-containingmaterial) is provided adjacent at least a portion of the substrate 110.According to a preferred embodiment, the carbon layer 115 may beprovided as a suspension of carbon or graphite powder in alcohol (e.g.,methanol, isopropanol, etc.), and may be provided in either apolymerizable or non-polymerizable form.

The carbon layer 115 may be deposited or formed by any suitable means.According to a preferred embodiment, the carbon layer 115 may beprovided using a spray gun or an equivalent alternative device. Othermethods for providing the carbon layer 115 may also be used according toalternative embodiments (e.g., sputtering, chemical vapor deposition,physical vapor deposition, etc.). The particular deposition method maybe chosen based on a variety of factors, including cost,manufacturability, and desired characteristics for the depositedmaterial.

According to a preferred embodiment, the carbon layer 115 includesgraphite particles having particle sizes of approximately 1 micrometer(e.g., between approximately 0.1 and 2 micrometers). One nonexclusiveexample of such material is commercially available as a graphite,colloidal, lubricant, aerosol spray by Alfa Aesar of Ward Mill, Mass.The carbon material is provided as a suspension of graphite inisopropanol. According to alternative embodiments, other types ofalcohol may be used in place of or in addition to isopropanol.

According to an exemplary embodiment, the carbon layer 115 includesmultiple layers of carbon-containing material that are deposited inmultiple deposition steps. For example, the carbon layer 115 may includebetween 3 and 20 layers of carbon-containing material and may have athickness of between approximately 20 and 30 micrometers. The number oflayers and the thickness of the carbon layer may vary according to avariety of alternative embodiments.

As shown in FIG. 8, in a step 240 (FIG. 10), the substrate 110 and thecarbon layer 115 are heated to a temperature of between approximately800° and 1000° C. at a pressure of approximately 10E-6 Torr forapproximately 1 hour (e.g., between approximately 30 and 90 minutes).During this vacuum baking step, alcohol provided with thecarbon-containing material is evaporated and/or pyrolized. At least aportion of the carbon atoms included in the layer of carbon material 115chemically react with metal atoms to form a carbide layer 120. Forexample, according to a preferred embodiment in which the substrate ismade of titanium, a titanium carbide layer is formed during the vacuumbaking step. The carbon atoms may displace oxygen atoms in the nativeoxide (e.g., titanium dioxide) formed on the surface of the substrateand/or may react with metal atoms included in the substrate. Where thesubstrate is provide as aluminum or an aluminum alloy, the vacuum bakingstep may be omitted.

The thickness of the carbide layer 120 may at least in part bedetermined by the amount of time the substrate 110 and carbon layer 115are heated in the vacuum baking step. According to an exemplaryembodiment, only a portion of the carbon layer 115 is consumed duringthe vacuum baking step, and a layer of unreacted carbon-containingmaterial 132 remains adjacent the carbide layer 120. According to analternative embodiment, the entire carbon layer 115 is consumed in thevacuum baking step and another layer of carbon-containing material maybe provided adjacent the carbide layer. The additional layer ofcarbon-containing material may have a composition which is the same asor different from that of the-carbon material used to form the carbidelayer.

In a step 250, the substrate 110, the carbide layer 120, and theunreacted carbon layer 132 is cooled to a temperature of betweenapproximately 20° and 100° C.

The substrate 110, the carbide layer 120, and the unreacted carbon layer132 are heated in a step 260 (FIG. 9) to a temperature of betweenapproximately 300° and 500° C. in an oxygen-containing ambient oratmosphere (e.g., air, pure oxygen, etc.) for a period of betweenapproximately 30 and 90 minutes. In this step, at least a portion of theunreacted carbon layer 132 is activated such that oxygen-containingfunctional groups such as CO, COOH, and C═O are created to form anactivated carbon region 134.

The carbonaceous layer 130 includes an activated carbon region 134 andan unreacted or non-activated carbon region 136 (which has a thicknessless than the unreacted carbon layer 132 according to an exemplaryembodiment. The unreacted carbon region 136 includes non-activatedcarbon-containing material. According to an alternative embodiment, theentire unreacted carbon layer 132 is converted to activated carbon suchthat there is no unreacted carbon left in the carbonaceous layer 130.

The relative thicknesses of the activated carbon region 134 and theunreacted carbon region 136 are a function of the amount of time thatelapses during the activation step. According to an exemplaryembodiment, the thickness of the activated layer is betweenapproximately 40 and 50 micrometers after heating at approximately 450°C. for approximately 30 minutes.

In a step 270, a solution of manganese nitrate (Mn(NO₃)₂) is introducedto the activated carbon region 134 of the carbonaceous layer 130.Because the activated carbon region 134 is relatively porous, thesolution penetrates into at least a portion of the carbonaceous layer130 (i.e., the solution infiltrates at least a portion of thecarbonaceous layer 130).

The manganese nitrate solution includes manganese nitrate and an alcoholsuch as ethanol, methanol, or isopropanol. Water is included in thesolution. According to a preferred embodiment, the solution comprisesbetween approximately 5 and 50 weight percent manganese nitrate, betweenapproximately 25 and 75 volume percent water, and between approximately25 and 75 volume percent methanol. According to alternative embodiments,the proportions of the various constituents of the solution may bevaried (e.g., water may be eliminated from the solution).

In a step 280, the substrate 110, the carbide layer 120, and thecarbonaceous layer 130 (including the manganese nitrate solution) areheated to a temperature of between approximately 150° and 350° C. in anoxygen-containing atmosphere for a period of between approximately 3 and90 minutes. According to other exemplary embodiments, a temperature ofup to approximately 400° C. may be used. Prior to this step, apre-treatment step (e.g., pre-drying or related treatments as are knownto those of skill in the art) may be performed. The environment in thepyrolysis chamber in which such steps are performed may include watersteam or related agents as are well known. According to a preferredembodiment, the manganese nitrate solution is heated to a temperature ofapproximately 250° C. for approximately 60 minutes. As a result of thisheating step, all or nearly all of the manganese nitrate is converted toan oxide of manganese (e.g., manganese dioxide). The oxide is providedas finely dispersed particles embedded in the activated carbon region134.

The oxide acts to further enhance the capacitance of the electrode 100.According to an exemplary embodiment, the capacitance of the electrode100 is greater than approximately 10 milliFarads per square centimeter.According to another exemplary embodiment, the capacitance of theelectrode 100 is greater than approximately 20 milliFarads per squarecentimeter.

FIG. 11 shows a graph 300 illustrating capacitance over frequencyfunctionality for electrodes having a carbonaceous electrode coatingsprovided over a titanium substrate. Both the abscissa 302 and theordinate 304 are provided as logarithmic scales. A first curve 310illustrates capacitance versus frequency functionality for an electrodehaving nine coats of carbon provided over a titanium substrate using asuspension including graphite in isopropanol vacuum baked at 800° C. andactivated at a temperature of 450° C. for one hour. A second curve 320illustrates capacitance versus frequency functionality for an electrodehaving four coats of graphite and enriched with manganese dioxide. Themanganese dioxide is provided by introducing four coats of manganesenitrate dissolved in methanol and water (50 percent water, 50 percentmethanol) and heating to 250° C. for 45 minutes in air. As shown in thegraph 300, the capacitance of the electrodes enriched with manganesedioxide is greater than that of the electrode including only activatedcarbon.

One advantageous feature of providing manganese nitrate in solution withan alcohol to form a capacitance-enhancing carbonaceous layer enrichedwith an oxide of manganese is that degradation or damage to theunderlying carbide layer and/or substrate is reduced or eliminatedduring processing. Such advantage may also be obtained when utilizingother nitrates or metal oxide precursors.

FIG. 12 shows a micrograph (magnified to 10×) of a manganese-dioxideenriched carbonaceous layer provided over a titanium substrate and atitanium carbide layer. The carbonaceous layer was prepared by utilizinga manganese nitrate and water solution. As shown, use of such a solutionproduced a ruptured or damaged carbonaceous layer. It is believed thatthe damage to the carbonaceous layer was a result of anelevated-temperature reaction between the manganese nitrate and thetitanium substrate underlying the titanium carbide layer during theformation of manganese dioxide. The result is a ruptured and damagedcarbonaceous layer.

FIG. 13 shows a micrograph (magnified to 10×) of a manganese dioxideenriched carbonaceous layer prepared by utilizing a manganese nitrate,water, and methanol solution. Providing alcohol in solution with themanganese nitrate produced a relatively smooth, high-capacitancecarbonaceous layer that does not exhibit the same defects found when amanganese nitrate and water solution were utilized (FIG. 12). In thismanner, the alcohol provided in solution with the manganese nitrate actsas a reducing agent which reduces or eliminates the reaction between themanganese nitrate and the underlying metal substrate layer. As a result,a relatively smooth and defect-free carbonaceous layer may be produced.

As utilized herein, the terms “approximately,” “about,” and similarterms are intended to have a broad meaning in harmony with the commonand accepted usage by those of ordinary skill in the art to which thesubject matter of this disclosure pertains. It should be understood bythose of skill in the art who review this disclosure that these termsare intended to allow a description of certain features described andclaimed without restricting the scope of these features to the precisenumerical ranges provided. Accordingly, these terms should beinterpreted as indicating that insubstantial or inconsequentialmodifications or alterations of the subject matter described and claimedare considered to be within the scope of the invention as recited in theappended claims.

The construction and arrangement of the elements of the capacitor asshown in the preferred and other exemplary embodiments is illustrativeonly. Although only a few embodiments of the present inventions havebeen described in detail in this disclosure, those skilled in the artwho review this disclosure will readily appreciate that manymodifications are possible without materially departing from the novelteachings and advantages of the subject matter recited in the claims.Accordingly, all such modifications are intended to be included withinthe scope of the present invention as defined in the appended claims.The order or sequence of any process or method steps may be varied orre-sequenced according to alternative embodiments. Materials or layersthat are “adjacent” or “proximate” each other may be in contact witheach other or may be separated by another material or layer, or by aplurality of such materials or layers. Other substitutions,modifications, changes and omissions may be made in the design,operating conditions and arrangement of the preferred and otherexemplary embodiments without departing from the scope of the presentinvention as expressed in the appended claims.

1. An electrode for an electrolytic capacitor comprising: a substratecomprising titanium; a carbide layer adjacent the substrate; and acarbonaceous layer adjacent the carbide layer and including means forenhancing the capacitance of the electrode.
 2. The electrode of claim 1,wherein the means for enhancing the capacitance of the electrodecomprises activated carbon.
 3. The electrode of claim 2, wherein theactivated carbon includes functional groups that include oxygen, thefunctional groups being selected from CO, COOH, and C═O.
 4. Theelectrode of claim 2, wherein the means for enhancing the capacitance ofthe electrode further comprises an oxide of manganese.
 5. The electrodeof claim 1, wherein the carbide layer comprises titanium.
 6. Theelectrode of claim 1, wherein the electrode is a cathode for anelectrolytic capacitor.
 7. The electrode of claim 1, wherein theelectrode has a capacitance of greater than approximately 10 milliFaradsper square centimeter.
 8. The electrode of claim 1, wherein thesubstrate has a roughness of between approximately 2 and 5 micrometers.9. The electrode of claim 1, wherein the thickness of the carbide layeris less than approximately 10 nanometers.
 10. A capacitor comprising: acathode; an anode; and an electrolyte provided intermediate the cathodeand the anode; wherein the cathode comprises: a substrate comprisingtitanium; a carbide layer adjacent the substrate; and a carbonaceouslayer adjacent the carbide layer comprising a material for enhancing thecapacitance of the electrode.
 11. The capacitor of claim 10, wherein thematerial for enhancing the capacitance of the electrode comprisesactivated carbon and an oxide of manganese.
 12. The capacitor of claim10, wherein the carbide layer comprises titanium.
 13. The capacitor ofclaim 10, wherein the cathode has a capacitance of greater thanapproximately 10 milliFarads per square centimeter.
 14. The capacitor ofclaim 10, wherein the substrate has a roughness of between approximately2 and 5 micrometers.
 15. The capacitor of claim 10, wherein thethickness of the carbide layer is less than approximately 10 nanometers.16. An implantable medical device comprising: an electrolytic capacitorhaving a cathode, an anode, and an electrolyte intermediate the cathodeand the anode; wherein the cathode comprises: a substrate comprisingtitanium; a carbide layer adjacent the substrate; and a carbonaceouslayer adjacent the carbide layer comprising a material for enhancing thecapacitance of the electrode; wherein the implantable medical device isconfigured to provide a therapeutic high voltage treatment.
 17. Theimplantable medical device of claim 16, wherein the material forenhancing the capacitance of the electrode comprises activated carbonand an oxide of manganese.
 18. The implantable medical device of claim16, wherein the carbide layer comprises titanium.
 19. The implantablemedical device of claim 16, wherein the cathode has a capacitance ofgreater than approximately 10 milliFarads per square centimeter.
 20. Theimplantable medical device of claim 16, wherein the thickness of thecarbide layer is less than approximately 10 nanometers.